(a) Field of the Invention
The present invention relates to a retrofocus-type objective for an endoscope and, more particularly, to a retrofocus-type objective for an endoscope of which aberrations are corrected favourably in all of the states that said objective is focused, i.e., the state focused on an object at a short distance, state focused on an object at a long distance, and respective states of focusing on objects at intermediate distance.
(b) Description of the Prior Art
An endoscope should have such overall structure that liquid will not enter the inside thereof. Therefore, an object for an endoscope is so arranged that the front lens thereof serves also as a cover glass. This means that, for an object for an endoscope, it is impossible to adopt the focusing method to advance the lens system as a whole or focusing method to advance the front lens through those focusing methods are widely adopted for lens systems in general. As a method devised in order to solve the above-mentioned problem, there is a known focusing method to keep the front lens group fixed and to move the rear lens group.
An objective for an endoscope disclosed in U.S. Pat. No. 4,059,344 is known as an objective for which the above-mentioned focusing method is adopted. Said known objective is designed as a retrofocus-type objective which comprises a front lens group comprising a diverging lens L.sub.1, and a rear lens group comprising converging lenses L.sub.2, L.sub.3 and L.sub.4, and which is arranged to be focused by moving the lens L.sub.2 along the optical axis, said known objective being suitable for the use with a device which constructed so that liquid will not enter the inside.
However, said known objective has disadvantages as explained below. That is, though aberrations thereof are corrected favourably when said known objective is focused on an object at a long distance, it is difficult to say that aberrations when focused on an object at a short distance are corrected satisfactorily favourably, i.e., astigmatism is undercorrected. Therefore, the resolving power in the marginal portion is not satisfactory.
Besides, in recent years, it is strongly desired to observe an object by enlarging it as far as possible at the time of observation by an endoscope. Therefore, also in the field of objectives for endoscopes, there is an increasing demand for a lens system which can be focused on an object at the infinite distance up to an object at an extremely short distance that corresponds to the object distance at the time of photographing by a camera using a macro lens. From the above-mentioned view point, the known objective according to the afore-mentioned patent has a disadvantage that it cannot be focused on an object at an extremely short distance at which the object can be observed with a satisfactorily high magnification.